How to understand Motorcycle Tires:
Sizes, speed & weight ratings, belt technologies, arc-design, etc...

PURPOSE:
 
   Although it may seem obvious to you that the tire's purpose is to hold the motorcycle up, the tires actually perform several different functions. They help maintain balance of the bike while in motion, they change the steering and handling geometry, they absorb some of the shocks and vibrations from the road surface (without involving the rest of the suspension), and the more obvious transference of energy between the bike and the road (braking, turning forces, acceleration) -- as well as handling the disappation of heat from that frictional transaction of energy transference.
 

TIRE COMPOUNDS:
 
   Tires come in different compounds for different intended uses. Softer compounds are designed to give an edge in grip (especially when the bike is leaned over significantly), while harder compounds are designed to promote longer tread life and cooler running (for long-haul driving). Most tire manufacturers now manufacture several different compounds to create a whole range of compounds available, from a super-soft tire specifically for track purposes, to a semi-soft street-racing tire, to a only-slightly-soft road touring tire. Additionally, some of the leading manufacturers also use different compounds within a single tire, such as the Metzeler Z-series, which uses a softer compound at the tire's outer edges and somewhat harder compound in the center, for a design that promotes exceptionally high grip while leaned over and yet still wears slowly in the center (for longer general lifespan).
 

TEMPERATURE VS. FRICTION:
 
   As we mentioned previously, one of the features of a tire is to handle the friction interface between the vehicle (motorcycle in our case) and the road, and then deal with the heat from that friction. If you've ever seen a semi-truck's tire tread laying by the side of the road, you've seen what happens to a tire that gets too hot (the molecular compounds inside the tire break apart, thus splitting or shredding the tire carcass). As you might imagine, having that happen on a motorcycle you are riding on is a sure-fire recipe for disaster.
 
   For motorcycle tires, there is a delicate balance going on here, because tire manufacturers want tires to get warm fast (their traction is dependent upon them "coming up to temp," or reaching a specific operating temperature -- and staying there), but at the same time don't want the tire to get too hot (or the tire material will start to wear out at a very fast pace -- or worse yet, delaminate itself as in the truck tire example above). The formulation of the tire compound plays a big part in this equation, since softer materials by their nature flex more (thus come up to "temp" faster, but also run hotter), while harder compounds flex less (thus take longer to get warm, but also run cooler). Since tires used on race tracks need to provide maximum traction, but are not expected to be used for more than a few races normally (a single race at pro levels), they are made of super-sticky materials designed to warm up very fast, and to run a bit hotter than normal street tires (trading life expectancy for superior grip). Touring tires, on the other hand, are expected to be able take a thousand miles or more in a day, for days on end, so they are built to heat much less quickly, and to run at a cooler temperature through-out their use (both for the lifespan of the tire, as well as for the stability of the tire carcass). Typical street tires fall somewhere inbetween these two standards, offering a blend that warms up faster than touring tires but runs slightly hotter than them too.
 
   Your riding style needs to play heavily into tire choices. Running a pure-sports tire, such as a Pirelli Diablo Corsa or Metzeler M3 or M1, without the constant on-brakes & change of direction input it needs in order to be kept in it's ideal operating temp range will means a seriously loss of traction compared to running a sports-touring tire that doesn't need that kind of input... In the same sense, running a sports-touring tire if you use the bike solely for track day sessions or are constantly knee-down canyon-carving is insanity. The right tire for the right task makes a huge difference in everything from stopping distances to outright grip-on-demand.
 
   The belt design plays into the temperature equation heavily as well, since adjacent belts can rub against each other, and do generate heat from flexing as well. Depending on the material the belts are manufactured of, the heat may spread out more evenly across the tire surface (example: steel belts spread heat very well, while nylon belts don't, because heat travels better through steel than nylon).
 

TREAD DESIGN - LAND AND SEA:
 
   Race tracks are pieces of exceptionally well maintained pothole-free road surfaces, with special crews that clean up oil spills and wipe off rain water with squeegies -- and thus tires for racing use often do not have any tread design at all or one with minimum treading (to maximize the rubber-to-ground interface).
 
   By comparison, a modern street tire has to deal with a long list of possible road-surface contaminants and irregularities, including water (rain, snow, slush), oil, automatic transmission fluid (ATF), spilled diesel fuel, sand, dirt, soil, gravel, loose asphalt, ridged concrete, slab-gaps and expansion-fills, bumps, pits, potholes, reflectors, cracks (and grass growing from the cracks), tar patches, etc. The list is almost endless. To help compensate for these irregularities, the carcass design of street/touring tires are different from those of track tires, including the inclusion of tire treads (in the USA, DOT-mandated). Please note that not all treads are designed equal!
 
   In the tire industry, the grooves cut or formed in tires are called "sea" and the ridges that border the grooves and the raised sections between the grooves are called "land". Different manufacturers have different land and sea designs, and even vary the land-sea design between tires in their own line-ups based on the intended use. Note that some designs have specific advantages over others.
 
   For example, if the front tire has grooves that go around the very center of the tire for it's entire circumference (center line groove/grooves), then the tire's center-line groove's sea area will have a tendency to track concrete that has ridges running in the same direction as traffic, or the metal grating on open-grated bridges (like some draw bridges) -- and can induce a very severe front end wobble as a result, which is disconcerting to say the least. To avoid this, many manufacturers redesigned the center line to go back and forth across the tire slightly (an oscillating center line design), while others avoid a centerline groove all together.
   It may be noted that outside of North America, I've never run into a piece of road work with concrete ridges running in the same direction as traffic, so it may not be a design concern in much of the rest of the world. On the other hand, centerline grooves provide a benefit when riding in heavy rains, slush and slurry, as they help maintain road contact. It's a trade-off, depending on where you live and what kinds of roads you tend to face. In the USA, I personally avoid tires with a straight centerline groove at all costs because of the tracking issue.
 
   Similarly, tires designed specifically for off-road use will have tread designs with wide seas and very large land-to-sea height differences to compact loose soil and sand under the tire to maximize traction on that kind of surface. Enduro tires, which are designed to be used on packed dirt roads and other semi-improved surfaces will have yet another design over those of conventional street tires, designed to increase the foot-print of the tire to the road to help offset slippage.
 

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Land and Sea Interactions:
   Motorcycle tire manufacturers design their land and sea areas to work together between the two tires (front and back), especially on street and wet-track tires, so that in rain or other contaminants, the front tire will create a partial path for the rear tire to track through -- and land/sea area of the back tire will only have to move what the front tire's tread didn't. This is the primary reason why no tire manufacturer recommends mixing different brands and/or designs of tires together -- because one manufacturer's front tire may not work well with another manufacturer's rear tire, and thus leave the rear tire with the tendency to aquaplane, or otherwise act up, thus risking your neck.

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BELT TECHNOLOGY:
 
   At one point, it was figured out that for road and racing tires, the inclusion of belt(s) of nylon tended to provide several benefits over belt-less tires. Nylon belted tires are called "Bias" tires. As the years went by, new materials (such as steel, kevlar, etc.) were tried in belt construction and those that were more effective in a specific use catagory were used as replacements for the older materials in that type of tire. Tires using such metal belts are called "Radial" tires, and are now stock and specified for virtually all newer motorcycles. Although officially bias and radial tires are method of winding the treads, in the motorcycle industry, bias tires use non-metallic belts and radials use metallic-basis belts.
   Older motorcycles can also often switch over to radials to gain the advantages they offer. A few of the benefits of radial tires gain by use of belts in motorcycle tire design are:

• Impact absorbtion: The tire spinning at speed throws the belt outwards (centrifical force), and this helps keep the tire's circumferance truely round, as well as helping it suck up minor bumps and dips without excessively stressing the rubber compounds, resulting in cooler running tires (and higher tire life).
   
• Temperature Distribution: Most metallic belting materials (such as steel) help distribute heat away from the point of friction (the place where the tire is contacting the road surface), across the entire surface of the tire, helping it both come up to temperature faster, and simultaneously run cooler at the point of contact, thus improving both performance and expected lifespan of the rubber-mix compounds used in the carcass.
   
• Reduced Rolling Resistance: Most belt materials help reduce the rolling resistance of tires by helping them hold their shape and giving them a certain amount of rotational mass.
   
• Lateral Stiffening: The presence of belts in a motorcycle tire's construction helps take the lateral stress load of the tire when it's running off-center (i.e. - when you are leaned over), so the rubber compounds don't have to handle as much of the force.
   
• Pressure-Loss Rate & Deformation Reduction: Radial tires have another distinct and major advatange here -- when they get compromised (punctured by a nail, screw, etc), they normally leak much slower than bias tires, which gives you more time to come to a controlled stop. Additionally, they tend to hold their shape much better while deflating at speed, which is crucial if the tire gets compromised while you are negotiating a turn, especially a high-speed turn. It can make the difference between walking away and leaving the scene in an ambulance, helicopter or hearse.

  Belts these days are woven in different patterns for different purposes. For example, a zero-degree pattern (with outer layer of threads running the same direction as the tire's rotation) provides a very high degree of high-speed stability. A diagonal belt pattern by comparison, provides a higher degree of lateral stiffness against tire deformation during cornering. Generally, tires designed for your specific kind of application (supersport/racing, sports/street-performance, sports-touring, pure-touring, enduro, etc) will have a belt design optimized for the conditions the tire is expected to see. Additionally, belt density and winding patterns are critical factors in the wieght handling characteristics of the tire -- a tire designed for a 700 lb bike capable of 150 MPH will traditionally have a denser spread of belting than a tire built to handle a 400 lb bike with a 100 mph top speed.

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Steel vs. Nylon:
   Although nylon is, by volume, a lighter material than steel, tires using steel belts may weigh less than similar designs using nylon, even with the same amount of other materials. Why? Because for the same total belt strength, a much thinner steel wire can be used in place of a thicker nylon strand in the belt construction.

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CHEMICAL COMPOSITION:
 
   Motorcycle tires were originally built out of pure vulcanized natural rubber. The industry has come a long, long way since then, and have added various bits of chemistry into their tires to change the characteristics of the tires. Exact tire compound formulations are now closely guarded trade secrets for the most part, but that doesn't mean that we can't look at the basic compounds and explain how they change the tires.
 
   The first compound is always the rubber base-compound everything else is added to, and these days it's usually synthetic rubber (which is consistant in it's strength and uniform in it's purity). Some manufacturers still add a small percentage of real rubber to their tires (usually for cost-effectiveness reasons), and a couple actually use significant amounts of natural rubber (the cheapest tires on the market).
 
   After that, there are two other major chemical compounds in use: carbon black and silica. Carbon black gives the compound it's black color and it's strength, while silica improves grip under all conditions (but especially wet ones). Then come the minor chemicals -- anywhere from 8 to 45 of them, including cobalt salts, various artificial resins (both designed to improve adhesion of various layers to each other), anti-oxidants (to improve the shelf-life of the tire), plus various other chemical compounds to improve different aspects of the tires. Most manufacturers pre-mix their rubber compounds before they hit the tire forms, but some of the newest motorcycle tire forming systems actually compose the chemicals on the spot at the tire forming machines.
 
   Now, let's go back to those two major chemical additives (carbon black and silica). By varying the percentages of these two chemicals in the formation of the tire's compounds, manufacturers can vary the hardness or softness of the tire, as well as the tackiness or grippiness of the tire. Replacing most the carbon black with silica reduces the hardness of the tire but promotes strength between the rubber molecules at the same time (and silica runs cooler than carbon black). But, removing most of the carbon black also reduces the lifespan of the tire, and may prompt the tire manufacturer to make a thicker tire to offset the removal of carbon black. Track tires are traditionally high in silica and low in carbon black compared to street tires, because grip is of paramount importance in track environments (and lifespan isn't, since most races are limited to 80 to 150 miles in length).
 
   Lately, a number of manufacturers have also been varying the initial bead size of these particles to improve the evenness of the mixture. Most tire manufacturing processes start with the raw mixtures preformed as small beads which will be joined in a heated press to form the tire layers in the press. R&D at these firms have found that they get new material matrix formulations by stepping to a smaller bead size, which can provide benefits in seeming contradictory values -- both better grip and longer life than prior generations of tires that used the same chemical basis in a larger bead size.
 

PROFILE DESIGN:
 
   Although we've talked about the grooves cut into a tire, we haven't discussed the actual profile of a tire (the outer curve of a tire when you look at it head-on). Different tires have different profiles, and generally speaking, a more triangular radius (think pointier) tire will handle faster, while a tire with a wide slow arc will handle more slowly but provide more straight-line stability (because of a larger center section to track flatly on).
   Manufacturers have been tweaking these parameters for decades to get benefits. Personally, I like Metzeler's solution (also implimented on some Pirelli tires as well) of a triple-radius design, which instead of having a single arc shape, actually has three arcs, each at a different curvature (so you get the benefit of the stability in the very center, with progressively faster handling as you lean further over).
 

FITMENT SIZE:
 
   Your rims are a certain size across from edge to edge (the short way), and specific diameter. The tires rated for your bike are designed to fit into this wheel (think "tire retainer" instead of the term wheel), while maintaining proper contact at the edges to keep the tire just rigid enough, not distorting the profile design (by rebending it to fit a narrower or wider gap at the base), and still clear everything else that the tire might rub on (such as the swing arms, the chain, etc).
 
 
How to read tire fitment charts and info:
 
Typical complete tire description: 120/70ZR17J (58W)
 
Now, let's break that down into pieces that make sense:

• The first number (120) represents the widest point of the tire's width between the left and right sides, called the "section width", and measured in millimeters. In this example, the width of the tire is 120 millimeters.
   
• The slash ( / ) is there to differenciate the first number from the second number, and serves no other purpose.
   
• The second number (70) represents the sidewall height as a percentage of the first number (in this example, it tell us the tire sidewall height is 70 percent of 120 millimeters, or 84 millimeters tall).
   
• Then comes either one or two letters (in our example, ZR). Sometimes these letters are placed elsewhere in the description, but traditionally, they should be listed this point. The first letter is always the speed rating (see Speed Rating chart), and the second letter, if it is there, is an "R" (radial tire) or a "B" (bias tire). Lack of a second letter means it is a bias or non-belted tire.
   
• The next numbers (17) represent the wheel's diameter, measured in inches. In this example, the wheel is 17 inches across.
   
• A letter at the end of the first part of the designation (the J in our example) is not required, but if present means that the tire has a special characteristics to match some manufacturer's specific motorcycle model. For example, Metzeler makes (made at the time I first wrote this portion) a Z4 radial as a 150/70ZR17, as a 150/70ZR17B, and as a 150/70ZR17J, where the "B" model was intended specifically the OEM factory tire for BMW 1150GS models (slightly different tread pattern), and the "J" model was intended for Yamaha FJR1300's (again a minor tread pattern variation from what I could tell).
   
• A two digit number follows, which is sometimes omitted but never should be (58). This number represents the load capacity or weight rating of the tire (in terms of how much weight, including both the rider and the motorcycle itself, the tire is designed to handle as it's maximum). In our example, the 58 can be looked up in a cross-referenced chart, and represents a maximum load rating of 520 lbs. There is no simple way to directly convert the number to the weight that I have been able to figure out.
   
• Finally, a letter may follow the load capacity (W). This is a manufacturer "modifying descriptor" and means that the standard for the tire is modified by some means (W generally means higher speed rated than a standard Z rated tire). We haven't been able to find a cross-reference for the various modifiers (yet -- they do vary by manufacturer), so if you are concerned about the rating modifier, contact the actual manufacturer's rep for the tire.
   
• Additionally, all motorcycle tires are normally marked with three or four digit code on their sidewalls, which represent the date of manufacture (the first two digits are the week of the year, the last digit is the year of the decade it was manufacturered in). Since modern cycle tires are only good for about five years from the date of their manufacture (the time period it takes the various compounds that keep the tire pliable and strong to evaporate out), knowing which year of a decade it was manufacturered is normally enough. Example: 011 would be a tire manufactured in January (1st week) of 2001. 118 would be a tire manufacturerd in the 11th week (between the 13th and 19th of March) of 1998 (since 2008 hasn't arrived yet, as I write this). As a general rule, newer tires are better, and we recommend you buy tires manufactured within the past 12 months whenever possible (obviously, if you have a flat away from home, you will take whatever you can get). Do not buy a four or five year old tire -- and if you buy an old bike, immediately check the tire age before riding it!
   

SPEED RATING:

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Speed Rating vs. Used Tires
   Tire's speed ratings are established with new production tires. Tires that have been repaired (patched), abused (dyno-tested, used at a track day), or are worn down somewhat no longer qualify for their original speed rating. Keep that in mind when you try to figure out why the motorcycle's manufacturer specified a 149+ mph rated tire for a bike that only goes 130 mph max. It's for your own safety!

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• RATING RATED TOP SPEED MPH / KPH
• J 62 mph / 100 kph
• N 87 mph / 140 kph
• P 94 mph / 150 kph
• S 112 mph / 180 kph
• H 130 mph / 210 kph
• V 149 mph / 240 kph
• Z 149+ mph / 240+ kph
• SPECIAL NOTE: Tires with a 2" to 2.5" nominal section width (think choppers) are rated at 75 mph.

LOAD RATING:
 
   The importance of the load rating on a tire can not be overstressed. A tire that is overloaded beyond it's design limit will run hot, wear fast and may quite literally blow out on you. The load the tire is designed to bear also affects it's shape and construction -- if you think about it, a GoldWing tire is not going to look like a GSXR600 tire. The load rating the manufacturer sets as the spec for your bike takes into account extra loading for weight-transfer (loading up the front wheel during braking, loading up the back wheel under acceleration), so you can't simply go by the curb weight of the bike. Always check the specs for your bike before ordering tires, and make sure the tires meet or exceed the load rating requirements for that wheel on that particular bike. For example, if the load requirement for the front wheel is "58" (520 lbs max), then you can fit a 59 or 60 weight-rated tire in the right size on there with no issue, but should never fit a 57 or below!
 
TIRE LOAD-RATING CHART

WHAT ELSE TO KNOW:

• NARROW VS WIDE: Although wide tires are all the rage now, narrower tires handle better under most real world conditions.
   
• TIRE GROWTH: All tires will expand to some degree over time once they are mounted, both due to the pressure on them, and due to heating-cooling cycles. Street tires normally expand 3 to 8%, while race tires can expand up to 22%. Be cautious in mounting a tire that will barely fit your clearances; the tire may easily grow to exceed the available space once it gets up to temp.
   
• UNEVEN TIRE WEAR (#1): Because roads are normally crowned to permit rainwater drainage, and you consistantly ride on the same side of the crown (right side of the road in the most countries; left side in the UK, Japan, and certain former British colonies), under normal street riding, tires wear unevenly to one side across the peak of their center apex.
   
• UNEVEN TIRE WEAR (#2): Depending on a variety of factors, including the size of the land-sea boundaries, the compound composition, the ambient temperature and the road surface temps, tires may wear unevenly from the front to rear of the individual tread blocks. Tire pressure and how it relates to tire surface temperature can have a drastic affect on whether this kind of scalloping/cupping at the land/see boundaries occurs, although certain tires types do it more than others.
   
• HIDDEN LEAKING: Your tires leak. Period. Under pressure, tires leak out some of their pressure directly through the rubber compound of the tire each and every day, very slowly. Check your tire pressure before you ride anywhere for the day (usually referred to as measured "cold") and add air as required.
   
• PRESSURE DIFFERENCIALS: A ten degree (F) change in the ambient temp will result in a significant pressure change in the tires. If the weather cooled or got hotter since yesterday, check your tire pressure again before you ride.
   
• NAILS, SCREWS, TACKS: If you get a nail, screw or tack in your tire miles from home or the closest shop, you have a few choices. The wisest of them is to call a ride to tow you & the bike to somewhere where you can get a new tire, then pull the object once you get there and hope it didn't go all the way through. The second is to cut it off flush (if it isn't already) and ride it to a safe place slowly -- very very slowly (think walking pace), ready for a possible leak or blow-out. The third is to pull the item out and pray it didn't go through -- and if it did, you're not going anywhere without a tow. And the final way is to carry an emergency patch/plug kit, pull the item, plug it as necessary, then proceed directly to somewhere that sells replacement tires.
   
• TIRE AGE: Motorcycle tires should not be used after five years from date of manufacture. Tires contain Volitile Organic Compounds (VOC's) which help keep the rubber flexible and grippy; as tires age, the VOC's leach out. Certain conditions can accelerate this process, including extremely hot and excessively humid or dry environments, parking on certain forms of man-made carpeting, certain molds & mildew forms (which may accelerate or even cause what is commonly known as "dry rot"), as well as overheating the tires due to low tire pressure or hard use.
   

Additional readings:

Sport Rider: "The Rubber Plant", a visit to a Michilen Tire factory Dunlop Motorcycle Tires - FAQ & Tire Tips
Wheel weight issues and handling results